Patent GB-1007611-A (GB)
Title: Improvements relating to clutches
IPC: F16D37/02
Filed: 1962-07-27
Abstract: 1,007,611. Clutch control. ROBERT BOSCH G.m.b.H. July 27, 1962 [July 28, 1961], No. 28895/62. Heading F2L. A control system for an electromagnetic clutch for a vehicle comprises a monostable relaxation oscillator supplying operating current pulses to the clutch coil, means for providing control pulses at a frequency corresponding to the speed of the engine for operating the monostable relaxation oscillator from its stable blocked condition to an unstable operating condition the duration of which determines the duration of the operating current pulses and which is determined by a timer circuit, and an electric pulse storing apparatus connected in parallel with the input of the relaxation oscillator and serving, when the speed of the engine is increased to such a value that the interval between two successive control pulses is shorter than the duration of an operating current pulse, to store a control pulse until the previously initiated operating pulse has ceased. The control pulses are produced by a coil 12 of the vehicle lighting dynamo 11 which is connected through a line 39 to the input of the control system which comprises a monostable relaxation oscillator 40 (outlined in broken lines) and an electronic pulse storing apparatus 41. The monostable relaxation oscillator 40 comprises an input ttansistor 42 which is normally conductive, and a current amplifier transistor 43 and a power transistor 44 which are normally blocked. When a positive control pulse is delivered through the lead 39, a condenser 60 and a rectifier 61 to the base of the previously conducting input transistor 42, this transistor is blocked and renders the two transistors 43, 44 conducting. The collector current of the transistors 43, 44 is supplied to the clutch coil 32. A condenser 45 which was charged during the conductive phase of the input transistor 42, maintains the transistor 42 in its blocked condition, which determines the duration of the operating pulse, until the condenser 45 has been discharged through resistors 51, 53, 54. The intervals between the operating current pulses decrease with increasing speed of the engine until at a predetermined value the clutch engages. As the engine is speeded up the control pulses follow one another so rapidly that they occur within the same operating pulse. The pulse storing apparatus 41 takes up these pulses, which it cannot convey to the relaxation oscillator 40 for releasing a new operating pulse until the previous operating pulse has ceased. The pulse storing apparatus 41 comprises an input transistor 70 and an output transistor 71. The transistor 70 is kept in a current conducting state so long as the transistors 43, 44 of the relaxation oscillator remain blocked and no magnetizing current flows through the clutch coil 32. If a control pulse is produced in the interval between two operating pulses, it can only actuate the relaxation oscillator 40 and remains ineffective upon the pulse storing apparatus 41. If, however, a control pulse is produced while an operating pulse is being conveyed through the clutch coil 32 and through the transistors 43, 44 the base of the input transistor 70 has a strong positive potential, and the increase in potential produced by the control impulse is sufficient to block the input transistor 70. The output transistor 71 then becomes current conducting, having been previously blocked, and the collector current maintains the transistor 70 blocked after the control pulse has terminated. When the operating pulse terminates and the transistors 43, 44 return to their blocked condition, a condenser 89, which has been charged during the operating pulse, begins to discharge through a resistor 88 and the clutch coil 32. When the condenser 89 has discharged, the input transistor 70 again becomes current conducting, producing a strong potential drop across a resistor 80 and at the same time blocking the output transistor 71. The voltage of the previously charged condenser 81 is added to the voltage drop across the resistor 80 and raises the base potential of the input transistor 42 of the relaxation oscillator 40 and thus blocks the input transistor to produce a new operating pulse. Without this pulse storing apparatus every second operating pulse would be lost at these high speeds. The consequence of that would be that the clutch torque would be reduced to that corresponding to half the actual speed. As the control voltage is tapped from the lighting dynamo, the difficulty arises at low speeds that the control pulse lasts for a considerable time and there is the danger that not only will the relaxation oscillator 40 release an operating pulse but the pulse will be taken up in the pulse storing apparatus and automatically release the new operating pulse when the previous operating pulse has ceased. To prevent this a condenser 91 is connected between the collector of the power transistor 44 and the base of the output transistor 71 through a resistor 90. The condenser is charged to the operating voltage in the intervals between two operating pulses. As soon as the collector potential of the power transistor 44 increases at the beginning of an operating pulse, the condenser raises the base potential of the output transistor 71 and maintains the output transistor firmly blocked until the control pulse has completely died down. In this way the pulse storing apparatus remains blocked from the beginning of each operating pulse for a brief period corresponding to the longest duration of the control pulse. In a modification, instead of the condenser 91 for blocking the pulse storing apparatus at the beginning of each operating pulse, an additional transistor is employed having its base connected to the coil 12 and the collector of which is connected through a condenser to the inputs of the monostable relaxation oscillator and of the pulse storing apparatus.